Carboxylic acids such as acetic acid are used in large quantities as basic chemicals for industrial purposes such as solvents, raw materials and intermediates in several reactions.
Various industrial processes are known for the preparation of carboxylic acids such as acetic acid. For example it is known to prepare carboxylic acid by the carbonylation of corresponding aliphatic alcohol, such as preparation of acetic acid by the carbonylation of methanol with carbon monoxide. The prior art discloses a number of catalysts for the preparation of carboxylic acid, particularly acetic acid, by carbonylation process. Generally, homogenous catalysts are used in carbonylation reactions.
German Patent Nos. 902495, 933148, 2303271 and 2400534 disclose the use of cobalt complex catalysts for the catalytic conversion of alcohols to carboxylic acids at temperatures in the range of 250 to 300° C. and pressure of from 680 t0 700 atmospheres. These high temperatures and pressures were necessary to stabilize the active catalytic species during the carbonylation reaction of for example, methanol to acetic acid. The processes of these disclosures also required complex separation processes and suffered from low selectivity to the final acid product.
French Patent No. 157313, South African Patent No. 68/2174 and U.S. Pat. Nos. 3,689,533 and 3,769,326 disclose the carbonylation of methanol using noble metals from Group VIII of the periodic table such as rhodium, iridium and platinum in the presence of bromine or iodine compounds as promoters. German Patent Nos. 1941449 and 1939286, British Patent No. 1233121, U.S. Pat. No. 3,816,490 and South African Patent No. 68/2174 disclose the use of rhodium metal complex and iodine compound promoters in the carbonylation of methanol to acetic acid. These processes operate at lower pressures of carbon monoxide compared to the earlier processes using cobalt complex catalysts, and provide a better selectivity for acetic acid. U.S. Pat. No. 5,510,524, EP 728726A1, 752406A1, 849249A1 and 849250A1 disclose the use of iridium complex catalyst for the carbonylation of methanol to acetic acid. The catalyst system employed comprises an iridium catalyst, alkyl halide promoter and promoters such as rhenium, ruthenium, osmium, cadmium, mercury, zinc, gallium, indium and tungsten. These promoters were found to be beneficial in getting higher activity at lower water concentrations. However, the above processes use iridium and rhodium, which are expensive ingredients. The systems for recovery are extremely complex and increase the cost of the reaction.
U.S. Pat. Nos. 4,537,871; 4,540,811; 4,134,912 and 4,356,320 disclose processes for the carbonylation of methanol using nickel catalysts and iodide promoters along with triphenyl phosphine and promoters such as molybdenum, tungsten, chromium or tin compounds. These processes are carried out at lower temperatures and lower pressures. However, these processes show poor selectivity for acetic acid due to the formation of side products such as dimethyl ether and methane. As a result these processes are inferior when compared to earlier processes using rhodium catalysts. U.S. Pat. No. 4,902,659 discloses nickel complex catalyst with isoquinoline as a ligand. While a claim of high selectivity is made, the stability of the catalyst is not known.